1. Field of the Invention
The invention concerns disposable diapers that are degradable and is more specifically concerned with the biodegradability of their diaper-fastening means, namely, the pressure-sensitive adhesive fastening tabs, release tapes, and frontal target strips of such diapers.
2. Background Art
Disposable diapers have employed pressure-sensitive adhesive fastening tabs with paper backings that may be biodegradable. However, paper backings are not reliable unless treated to resist moisture and to afford good strength. Unfortunately, treatments that are effective may inhibit the degradability of the paper. Furthermore, the application of such treatments can make paper expensive. Also, paper is comparatively stiff, so that the tape may not flex with the diaper, and the edge of the tape might injure a baby's tender skin.
Accordingly, paper backings for diaper fastening tabs have been virtually supplanted by fastening tabs having backings of thermoplastic films, see, for example, U.S. Pat. No. 4,237,889 (Gobran). For teachings of making the fastening tabs tougher, see U.S. Pat. No. 4,808,474 (Sipinen); and for teachings of making the tabs soft, pliable, conformable, and heat sealable, see U.S. Pat. No. 4,769,283 (Sipinen et al). The fastening tab of each of these Sipinen patents has a backing of a substantially untensilized, ductile film of a blend of crystalline isotactic polypropylene and a compatible ethylene-based polymer in proportions found to make, e.g., fastening tabs tough or soft, respectively. Useful compatible ethylene-based polymers include an ethylene-propylene polymer and polymers of ethylene-vinyl acetate and ethylene-acrylic acid.
Current interest in disposable diapers has lead to the manufacture of diapers that are claimed to be mostly biodegradable. However, none known has a polyolefin backed degradable fastening tab. Further, tapes used to make fastening tabs are currently not degradable polyolefins. Thus, although the need for degradable fastening tabs, and associated tapes (i.e. frontal target strips and release tapes) exists, a suitable solution is not currently available.
U.S. Pat. No. 4,016,117 (Griffin) imparts biodegradability to a synthetic resin by blending it with natural starch as well as fatty material that apparently serves as a lubricating substance. In Example I, maize starch was blended with ethyl oleate, oleic acid, and low-density polyethylene to form a smooth compound which was diced into cubes that were about 50% by weight starch. The cubes were blended with additional low-density polyethylene to give 8% starch in a composition which was formed into a translucent flexible film that was biodegradable under laboratory conditions designed to simulate a landfill, as reported in Example II. The tensile and tear strengths of this film were only moderately lower than those of identical film except for omission of the starch and fatty material. Other examples of the Griffin patent employ, instead of the low-density polyethylene, polystyrene (Examples III and IV) and polyurethane (Examples XII-XIV). Polypropylene is mentioned at col. 3, line 22. U.S. Pat. No. 4,021,388 (Griffin) contains substantially identical disclosure. U.S. Pat. No. 4,324,709 (Griffin) says that natural starch can be more readily mixed with a polymer by pre-mixing a small amount of a fluid lubricating substance with the starch, e.g. in Example 1, starch was preblended with 10% of ethyloleate, an oily material, together with 4% of calcium stearate, another oily material.
Pellets, used like the cubes of Griffin ,117, are currently marketed as "Polygrade II" by Ampacet Corp., Mt. Vernon, NY. These pellets are based on linear low-density polyethylene and contain 36% by weight starch. The pellets also contain by weight about 0.4% of what is called a chemical prodegradant that is a metal salt plus about 13% by weight of an unsaturated elastomer. Ampacet literature states that these pellets can be blended with either linear or nonlinear low-density polyethylene to produce a film that is biodegradable when its starch content is from about 6 to 12% by weight, and suggests using the film to make garbage bags. The tensile and tear strengths of garbage bags as well as their method of manufacture makes these films unsuitable for use as diaper tapes.
St. Lawrence Starch markets "Ecostar" concentrates in the form of pellets that comprise by weight 52% linear low-density polyethylene, 43% corn starch, and 5% vegetable oil. St. Lawrence Starch also markets "Ecostar Plus" pellets that are believed to be similar to the "Ecostar" pellets except also containing a chemical prodegradant.
Therefore, it is a general object of this invention to solve the problems of the prior art by providing a suitable degradable polyolefin diaper fastening tab.